The present invention is concerned with control of motor vehicle powertrains, specifically those having a continuously variable transmission with the ability to provide an infinite speed reduction from input to output (“geared neutral”).
Continuously variable transmissions typically comprise a variator—that is, a device for providing a steplessly variable drive ratio—coupled between a transmission input and output through additional gearing. Some such transmissions are able to achieve a state referred to as “geared neutral” in which they provide an infinite speed reduction from input to output, so that even though the transmission input rotates and is coupled to the output through the transmission, the transmission output is stationary. One known type of geared neutral transmission uses an epicyclic shunt gear having three rotary shafts coupled respectively to sun, ring and planet carrier components of the epicyclic. One shaft is also coupled to the transmission input through the variator and so drivable therefrom at continuously variable ratio. Another of the shafts is coupled to the transmission input through a fixed ratio gear train. The third shaft is coupled through a further gear train to the transmission output and its rotational speed is a function of the speeds of the other two shafts, as those familiar with epicyclic gears wilt immediately appreciate. At a particular variator drive ratio, the motions of the first and second shafts cancel each other out, leaving the third shaft, and the transmission output, stationary.
By convention, transmission ratio is expressed as the transmission's input speed divided by its output speed. Expressed in this way the ratio behaves asymptotically, tending to infinity at geared neutral. Consequently it will be convenient in what follows to refer instead to the “reciprocal transmission ratio”—i.e. transmission output speed divided by input speed—whose behaviour around geared neutral is non-asymptotic (it simply falls to zero) and so easier to describe.
Of course most commercial motor vehicle transmissions rely upon some form of “launch device”, such as the driver-operated clutch of a conventional manual transmission, or the torque converter of a conventional stepped-ratio automatic, to de-couple the engine and vehicle wheels when the vehicle is stationary, and to allow slip between engine and wheels as the vehicle moves away. In a geared neutral transmission it is possible (although not obligatory) to dispense with the launch device. The transmission is able to move from reverse gear through geared neutral to forward gear simply due to changes in the variator drive ratio, without the need to de-couple engine and wheels. At the very low reciprocal transmission ratios made possible by a geared neutral transmission, problems arise in controlling the powertrain which are simply never encountered with more conventional transmissions.
In an ideal system with no frictional loss, compliance, etc., a zero reciprocal transmission ratio would imply an infinite torque multiplication from the transmission's input to its output. That is, any torque at the input would produce an infinite torque at the output. Of course, no practical transmission can provide infinite output torque. A more practical way to characterise the situation is to say that in this condition, in the steady state (i.e. when the engine/transmission input is not accelerating) the torque at the transmission input is zero whatever the torque at its output. A condition in which torque at the transmission output creates zero torque at its input will be referred to herein as a “singularity”. It has implications for the manner in which the engine and transmission should be controlled around geared neutral.
Certain consequent problems, and one method for addressing them, were discussed in Torotrak (Development) Limited's prior U.S. Pat. No. 5,521,819 and its European counterpart 643648. The present invention results, however, from a recognition not found in this earlier patent—that in a practical transmission, because of frictional losses, there are not one but two singularities, neither of which occurs precisely at geared neutral. Instead the two singularities occur at respective reciprocal transmission ratios close to geared neutral but lying to either side of it—i.e. at low forward and reverse reciprocal transmission ratios. The singularities are encountered only in “overrun” conditions—that is, where the torque exerted by the transmission at its output tends to decelerate the transmission output. In this condition power is input to the transmission through the output, and it is when this power is equal to the power dissipated by transmission inefficiency that the singularity occurs.
This understanding has important implications for the manner in which the engine and transmission are controlled. Existing control systems which operate flawlessly in most conditions have been found to malfunction severely under certain circumstances, such as when “shuttling” and hill climbing/descending. It is now recognised that it is in these situations that the singularities are encountered and must be allowed for.
On the other hand the advantages to be gained from proper powertrain management at ratios around the singularities, in accordance with the present invention, have been found to be startling. In particular it becomes possible to use the transmission to exercise such accurate low speed control of the vehicle as to make possible a whole range of control options for the driver.
One particular problem concerns control of engine speed in overrun. It is useful in this context to draw a distinction between (1) a situation where the engine is “loaded” by the transmission—that is, torque at the transmission input tends to slow the engine, and (2) the opposite condition where the engine is “unloaded”, the torque at the transmission input tending to drive the engine. Unloading of the engine occurs in overrun at ratios away from geared neutral. Power flows from the wheels to the engine and is dissipated frictionally in it, providing “engine braking”. In this condition, in an electronically controlled powertrain, it is conventional to de-fuel the engine, applying a zero torque demand to it. However, overrun does not always cause the engine to be unloaded. At reciprocal transmission ratios between the singularities, the engine is always loaded despite any overrun torque at the transmission output/vehicle wheels. Failure to take account of this leads to loss of control of engine speed in the exceptional conditions where the singularities are experienced.